This invention relates to television camera tube/coil assemblies in which the focus coil is coupled in a common circuit with an alignment coil to utilize the same current supply.
Tube/coil assemblies are often utilized for imaging in the electronic arts. For example, television cameras may use imaging tubes such as vidicon tubes and the like. Such tubes generally include a cathode for emitting electrons towards a target electrode upon which a visual image is focused by optical elements. The signal output is taken from the target electrode as a result of the interaction of the electron beam with the light responsive target. In order to have a television video signal derived from the target which properly represents the image, various distortions must be avoided and the resolution must be high. High resolution requires that the electron beam be focused on the target electrode. Such focusing is provided by a focus coil which is generally a solenoid winding extending over a substantial portion of the tube. Deflection windings are also included in such assemblies in order to deflect the focused electron beam over the surface of the target so as to scan a rectangular raster representing the image.
Among the constraints required in order to have the video signal properly represent the image being scanned is the requirement that the electron beam be orthogonal to the plane of the target during those times when it is directed on the center of the raster. If the beam direction at the center of the raster is not orthogonal to the surface, various distortions of the video can occur. Such distortions include lag, poor resolution, degraded picture geometry and signal anomalies referred to as "waterfall" or "stern wave" in which the plane of the image appears to be on a water surface that has ripples passing thereacross, such as waves on the surface of the ocean.
The electron beam originates from a cathode and passes through a grid having a small hole which is a defining aperture. That portion of the electron beam passing through the defining aperture may be travelling along a path which is skewed with respect to the longitudinal axis of the tube. The skew angle is the misalignment angle .phi. as illustrated in FIG. 1. It is known to correct the misalignment angle to substantially zero degrees by the use of a magnetic field generator in the region between the defining aperture and the beam-entrance end of the focus coil. It is known to use permanent magnets to generate a magnetic field in this region for alignment of the electron beam to reduce the misalignment angle .phi. to zero. The advantage of a permanent magnet alignment method is that it requires no electrical power to maintain the field. However, the field generated by permanent magnets is difficult to control. Since the magnitude of the required magnetic field will vary from tube to tube, it is necessary to either control the magnitude of a single permanent magnet or to provide two permanent magnets and use one to partially offset the field of the other so as to control the resulting magnitude of the magnetic field. Where a small resulting magnitude is required, the magnetic field magnitudes of the two permanent magnets must be identical. Such identical magnets are difficult to manufacture. Also, in those cases where a great deal of cancellation is required in order to achieve a small resulting field magnitude, the slightest relative motion between the two permanent magnets can create major changes in the resulting field magnitude. Consequently, such arrangements may be easy to adjust but are difficult to maintain in adjustment. Another disadvantage arising from the use of permanent magnets is that magnetic singularities in the magnetic structures can cause irregularities in the field derived therewith. Furthermore, for the permanent magnets to be physically positioned relative to each other in providing the appropriate resulting field and then the magnet combination to be positioned in correcting the alignment of the electron beam, required ready access to the magnets. This ready access tends to complicate the shielding by which the entire image tube is shielded from external magnetic fields. Thus, the use of permanent magnets to correct for beam misalignment angle presents many disadvantages.
In order to avoid the disadvantages of permanent magnets, it is known to use electromagnets. Such electromagnets are ordinarily operated as orthogonal pairs and a current through each is controlled both in magnitude and in polarity as as to develop a resultant field of the proper direction and amplitude to correct the beam misalignment angle. However, this requires at least a regulated current source and a control element for each winding. Consequently, a three-tube color camera might require as many as 6 individual coil windings. Since the current in the various coil windings are in general dissimilar, even as to polarity, no possibility exists for simultaneous control of those windings. Therefore, individual current sources and control elements would be necessary and consequently, substantial power dissipation and electrically complexity result. These are extremely disadvantageous, especially for those cameras which are used in portable applications where power must be provided by batteries. The complexity of the circuits and the numerous adjustments undesirably add weight, while the current load reduces the maximum time of operation. Obviously then, an electromagnet arrangement of reduced power consumption and having simplified circuitry would be extremely desirable for electric beam alignment.